Pressure vessels are commonly used for containing a variety of gases or fluids under pressure, such as hydrogen, oxygen, natural gas, nitrogen, propane and other fuels, for example. Generally, pressure vessels can be of any size or configuration. The vessels can be heavy or light, single-use (e.g., disposable), reusable, subjected to high pressures (greater than 50 psi, for example), low pressures (less than 50 psi, for example), or used for storing fluids at elevated or cryogenic temperatures, for example.
Suitable pressure vessel shell materials include metals, such as steel; or composites, which may be formed of laminated layers of wound fiberglass filaments or other synthetic filaments bonded together by a thermo-setting or thermoplastic resin. A liner or bladder is often disposed within a pressure vessel shell to seal the vessel, thereby serving as a fluid permeation barrier.
Generally, pressure vessels have limited lifetimes, and it is desirable to remove a pressure vessel from service before it fails, as failures can be catastrophic and cause damage or injury. Both cyclic fatigue and static fatigue (stress rupture) contribute to the fatigue load, and thus the failure, of pressure vessels. The calendar life of a pressure vessel, or the number of fatigue cycles over a specific pressure range (for example, from near empty to full), is commonly used to determine when to remove a vessel from service. However, in some applications, the pressure ranges and number of cycles applied to the pressure vessel are inconsistent and/or unknown. In addition, the interaction between cyclic fatigue life and static fatigue life is not well understood. The effects of cycling combine in unknown ways with the effects of the duration the pressure vessel spends at full pressure without cycling.
Mathematical projections of vessel lifetime are commonly used to evaluate the fatigue life of a pressure vessel. This requires that the number of cycles be counted or estimated, then sorted by mean stress levels and stress range. These cycles are combined into an equivalent number of full-range cycles to estimate the remaining vessel life. It must then be determined how to combine this information with static fatigue. Uncertainties are inherent in the calculation and estimation of cycles, in combining cycle effects, and in assessing the total and remaining life of the pressure vessel.